220 A. J. Jukes-Browne — Physiography of the Lower Trias. 



branches to Tylodendron is pointed out by Blanckenhorn, who agi'ees 

 with Weiss that the slits in the so-called leaf-cushions represent 

 resin canals. 



That these specimens described by Blanckenhorn are really casts 

 of medullary cavities of Voltzia heterophylla I have no doubt ; what 

 he took for leaf-cushions, and what Weiss regarded as such in the 

 case of Tylodendron, are the casts of the internal ends of the medul- 

 lary rays, and represent radial prolongations of the medullary tissue. 



V. — The Physiography of the Lower Trias. 



By A. J. Jukes-Browne, B.A., F.G.S. 



nnHE question of the physical conditions under which the Lower 

 I Triassic sandstones and pebble-beds were formed is certainly 

 one which should interest all readers of the Geological Magazine, 

 if only that it is strange that it should still be an open question 

 whether so important a formation is of marine or fluviatile origin. 



Perhaps a few words from one who takes a keen general interest 

 in the subject, but who has no special theory to defend, may help to 

 define the issues and to show how far the arguments adduced on 

 each side influence a reader who had no previous bias. Far be it 

 from me to pose as a judge or arbitrator. I only wish to write from 

 the "intelligent public" point of view. 



As Mr. Mellard Eeade opened the present discussion, I take his 

 arguments first, and three of them seem to be strong points : — 



1. The universal sandiness of the Lower Trias. Considering 

 that the deposit rests on and against Carboniferous Limestone, Mill- 

 stone-grits, Coal-measures and Permian rocks, it is curious that 

 pebbles derived from these rocks should be so rare in the Triassic 

 conglomerates ; where too has all the shaly material from the 

 Carboniferous series gone to ? These questions are greater difficul- 

 ties on the fluviatile than on the marine hypothesis. Prof. Bonney 

 does not attempt any actual explanation, but only cites analogous 

 cases from the Alpine regions. 



2. The existence of Lower Trias sandstone in the vale of Clwyd, 

 a valley that drains from south to north, is admitted as a difficulty 

 by Prof. Bonney. 



3. The greater thickness of the pebble-beds in the midland 

 counties and the diminution in the size of the pebbles toward the 

 north. This is certainly a valid argument against the theory that 

 the pebbles have been brought from the north, and so far as I am 

 aware it has never been fully met by Prof. Bonney. 



Let us now turn to the three points adduced by Prof. Bonney in 

 the Geol. Mag. for February as adverse to Mr. Beade's view ; and 

 of which, by the way, only one is discussed in Mr. Beade's rejoinder. 



1. The strength of the marine currents necessary to move large 

 pebbles in water of any depth. This is partially met in Mr. Beade's 

 reply, but the facts he quotes are not very convincing, and Mr. 

 Hunt's letter in the same number challenges the accuracy of his 

 inferences. The idea that the deep valley-like trench in the Irish 



